Isocyanate-functional polyurethanes or prepolymers of polyols and polyisocyanates are known and are the basis of many existing commercial products.
Reactive polyurethanes have reactive end groups which can react with water or other compounds which have an acid hydrogen atom. This form of reactivity makes it possible for the reactive polyurethanes to be brought in the desired manner to the desired location in the processed state (as a rule liquid to highly viscous) and to be cured by the addition of water or other compounds which have an acid hydrogen atom (in this case called curing agents).
In these so-called 2C systems, the curing agent is as a rule added directly before the application, in the normal case with the aid of a mixing and metering system, the processor having only a limited processing time (pot life) available after the addition of the curing agent.
However, it is likewise possible to cure polyurethanes with reactive end groups solely by the reaction with moisture in the atmosphere, without addition of curing agents (1C systems). Such 1C systems as a rule have the advantage over the 2C systems that for the user the often troublesome mixing of the frequently viscous component before application is omitted.
The polyurethanes with reactive end groups conventionally employed in 1C or 2C systems include, for example, the polyurethanes with preferably terminal isocyanate or NCO groups.
In order to obtain polyurethanes with terminal NCO groups, it is conventional to react polyfunctional alcohols with an excess of monomeric polyisocyanates, as a rule diisocyanates.
It is known that at the end of the reaction, regardless of the reaction time, a certain amount of the monomeric diisocyanate employed in excess is left over.
A content of monomeric diisocyanate has a troublesome effect, for example, during the processing of adhesives and sealants based on reactive polyurethanes. Even at room temperature, diisocyanates, such as IPDI or TDI, can have a vapour pressure which is not to be ignored. This noticeable vapour pressure has a serious effect in a spray application in particular, since significant amounts of isocyanate vapours, which have an irritant and sensitizing action, that is to say are toxic and consequently undesirable, may arise here via the application apparatus.
For many fields of use, the development of reactive polyurethanes or prepolymers with a drastically reduced content of monomeric diisocyanates is therefore highly desirable.
Reactive prepolymers or polyurethanes with a reduced residual monomer content can be prepared, for example, by the process disclosed in International Patent Publication No. WO1997046603, the entire contents of which are hereby incorporated herein by reference. In this, an excess of diisocyanate is reacted with polyols and the excess monomeric diisocyanate which remains after the reaction is removed by distillation. A disadvantage of this process is that products with high NCO contents typically have extremely high viscosities.
Alternatively, reactive prepolymers or polyurethanes with a reduced residual monomer content can be prepared in accordance with the teachings of, for example, DE-A 10 229 519 or DE-A 10 163 857, the entire contents of each of which are hereby incorporated herein by reference. In this case, diisocyanates with NCO groups of different reactivity are used and are reacted in a slight excess with polyol. A disadvantage of this procedure is that the resulting products typically have low NCO contents.
There is therefore still the desire to provide reactive prepolymers or polyurethanes which have low viscosities, high NCO contents and low residual monomer contents.
Higher molecular weight polyether-amines can be reacted with formic acid alkyl esters to give the corresponding formamide-terminated oligomers, which can be reacted further with polyisocyanates to give the corresponding prepolymers, that is to say to give acylurea prepolymers. These have a generally low viscosity.